The widespread introduction and acceptance of laser surgical systems in ophthalmic applications ushered in a new era of precision and control. One of the keys to achieving this high level of control is the immobilization of the eye relative to the laser surgical system. In many devices, the immobilization is carried out by affixing a patient interface to an objective of the laser and then docking it to the eye, often by vacuum suction. In other systems, a portion of the patient interface is docked to the eye, another portion to the objective, and then the surgeon gently aligns and locks the two portions together.
One of the factors the precision and utility of these systems depends on is the patient interface being docked to the eye in a central position. Such a central docking or centering can align an optical axis of the objective of the laser system and an optical axis of the eye. Since the laser beam is typically directed and controlled relative to the optical axis of the objective, aligning the optical axis of the eye with the optical axis of the objective by centering the docking can enable controlling the laser beam within the eye with high precision.
Centering the docking with the visible structures of the eye, such as the pupil or limbus is often a challenge, though, for multiple reasons. The patients sometimes move their eyes during docking, even against their own will. Also, even if the patient interface was centered with the eye at the beginning of the docking procedure, the globe of the eye can roll to one side during docking because of the pressure applied by the patient interface after contact has been made with the eye. Further, the shape of the eye structures can be an ellipsoid or irregular to some degree. Also, the limbus and the pupil are often not concentric. In these typical cases the center of the eye is not entirely well-defined: e.g. centering the patient interface with the pupil may not center it relative to the limbus.
An additional layer of complexity arises in systems intended for cataract procedures. The target of the cataract procedures is the lens, having limited visibility because it is an internal structure of the eye and it is essentially transparent. Moreover, the lens is typically not concentric with the visible structures of the eye, including the limbus and the pupil. For all these reasons, centering the patient interface with the limited visibility lens is hard. If the patient interface is centered with the visible limbus instead, this may also result in docking the interface misaligned with the limited-visibility internal lens. In this case, when during the cataract surgery the laser beam is referenced relative to the center of the patient interface aligned and docked with the limbus, the laser beam may be misdirected relative to the center of the lens, the intended target of the cataract surgery.
There can be several reasons for the lens being off-center. In many eyes the lens is anatomically off-center. Moreover, the pressure of the docking may also push and tilt the lens to one side as the lens is held in its place only by soft ciliary muscles.
Some systems compensate the lens being off-center by attempting to align the patient interface with the lens instead of the visible pupil. However, the transparency of the lens makes it difficult for the surgeon to determine the precise location and tilt of the lens and to align the patient interface accordingly.
Some systems employ an imaging system to image the lens to assist the alignment of the patient interface. However, the use of such imaging systems can encounter problems as well.